1. Field of the Invention
The present invention relates to a special functional carbon nano material such as a carbon material for storing and recovering a gas such as hydrogen or natural gas, a carbon material for separating and refining a liquid or gas, and a carbon material for a high capacity electrode of a cell or a super capacitor, and more particularly, to an ultrafine porous graphitic carbon fiber prepared by carbonizing an electrospun ultrafine fiber of halogenated polymers containing a graphitizing catalyst, and a preparation method thereof.
2. Description of the Background Art
Recently, a lightweight carbon material which can adsorb hydrogen or methane having a high vapor pressure in the normal temperature at a high concentration has drawn attention. An activated carbon adsorbs at least about 15 wt % of methane gas at 25° C. in 35 pressure. A methane natural gas is charged in a high pressure vessel made of non-rusted steel with 200 pressure and used for a natural gas bus. In addition, a fuel cell electric vehicle using hydrogen enters into a road driving test stage. When a storage pressure of the hydrogen or compressed natural gas increases, a storage ability also increases. However, since a strong heavy storage tank is mounted, a high density adsorption carbon material for hydrogen or gas is necessary.
Required are a high reliability high capacitance small-sized capacitor which is a backup power source of a small-sized electronic device, and a high capacitance capacitor for packet transmission of a digital camera memory or a cellular phone which can instantaneously supply power. Especially, for practical use of a hybrid vehicle or an electric vehicle, a high capacitance electric double layer capacitor (EDLC) is required as an auxiliary power source (power assist) of an internal engine (gasoline, diesel, LPG or CNG engine), a secondary cell for power supply or a fuel cell. The characteristics required in the carbon material for the capacitor electrode having high capacitance or the high density gas adsorption include a large specific surface area and high conductivity. A pore volume capacity, a pore micropore size distribution, a geometrical pore structure and a chemical pore property are also very important.
The size of the pores by the activated carbon is a critical factor of a gas adsorption process. Therefore, the size of the pores must be very similar to the size of the gas molecules to be adsorbed. For example, adsorption of methane having a molecular diameter of 0.38 nm is dependent upon the volume of the micropores of the activated carbon. In the pore size distribution of the used activated carbon, a pore size width of 0.6 to 1.0 nm occupies the largest volume. The effective pore size width of the activated carbon adsorption is 0.78 nm. The micropores are more advantageous in adsorption over a critical temperature than the macropores. A grain or powder phase material such as the activated carbon needs molding in use. Here, the performance of the material may be deteriorated due to an additive for shape stability such as a polymer binder. Conversely, a fibrous carbon material such as a carbon fiber or an activated carbon fiber does not have the foregoing problem. In the case of the fibrous carbon material, a size of pores can be easily controlled in carbonization. A thin diameter of a fiber is advantageous in formation of microporous carbon. On the other hand, the activated carbon or the activated carbon fiber is prepared by activating the carbon material to enlarge a specific surface area. The activation process enlarges the specific surface area, but also increases the macropores and the micropores. It is thus difficult to control the pore size distribution.
The carbon material prepared by carbonizing halogenated polymers such as poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl chloride), and copolymers thereof has a large specific surface area and a well-controlled pore size without the activation process, and thus draws great attention in the industrial or scientific fields. The carbon material can be efficiently used for high energy storage and electrochemical devices. Accordingly, if the ultrafine fiber of the halogenated polymers is used as a carbon fiber precursor, the prepared carbon material is expected to have very excellent performance. However, the ultrafine carbon fiber is not yet prepared due to low thermal stability and fiber forming ability.